1. Field of the Invention
The present invention relates to a display and a method of fabricating the same, and more particularly, it relates to a driver-integrated display comprising a display pixel part and a peripheral driver for driving the display pixel part which are formed on the same substrate, and a method of fabricating the same.
2. Description of the Background Art
In recent years, a semiconductor device having a semiconductor layer of polycrystalline silicon which is extremely higher in carrier mobility than amorphous silicon is watched with interest. Such a semiconductor device includes a driver-integrated liquid crystal display comprising a display pixel part provided with a plurality of display pixels and a peripheral driver for driving the display pixels, which are formed on the same substrate.
The conventional driver-integrated liquid crystal display is now described.
FIG. 15 is a general block diagram showing the conventional driver-integrated liquid crystal display. Referring to FIG. 15, the conventional driver-integrated display comprises a display region 28 provided with display pixels which are connected with thin film transistors (TFT) serving as display driving elements, and a peripheral driving circuit consisting of drain drivers 24 and gate drivers 25 for driving the display driving elements, which are provided on the same substrate 1.
FIG. 16 is a plan view showing a pixel part of the conventional display, i.e., the display driving elements provided in the display region and wirings between the elements. As shown in FIG. 16, drain lines 26 to which video signals are transmitted are positioned above gate lines 6 of intermediate wirings, to serve as upper wirings. Pixel electrodes 17 are formed in regions which are enclosed with the drain lines 26 and the gate lines 6. A storage capacitive electrode 27 is formed to overlap with each pixel electrode 17, in order to form a storage capacitance.
Further, TFT capacitances in the TFTs and cross capacitances between the drain and gate lines are present as parasitic capacitances. While the storage capacitances are largely designed with respect to the parasitic capacitances, compensation by the storage capacitances is made only in case of a voltage drop during electric interception of the TFTs.
The aforementioned point is described with reference to FIG. 17. FIG. 17 is an equivalent circuit diagram of the conventional display unit. The drain driver 24, the gate driver 25, and the display region occupying the remaining region are provided on the same substrate 1. The drain driver 24 is formed by a shift register 29 and a sampling transistor 30 which receives a shift pulse from the shift register 29 for sampling the video signal from each drain line 26. On the other hand, the display region of the liquid crystal display is formed by a TFT t1, a liquid crystal 23 to which the sampled video signal is applied from a source of the TFT t1, and a storage capacitance SC. Thus, the storage capacitance, which is provided on the liquid crystal region, is not adapted to compensate for potential change in conduction of the TFT.
In order to illustrate a portion forming the aforementioned parasitic capacitance, FIG. 18 shows a sectional view taken along each drain line 26 of the conventional driver-integrated liquid crystal display. Referring to FIG. 18, the drain line 26 electrically connects the sampling transistor 30 of the drain driver region with the TFT t1 of the display region. The parasitic capacitance is proportionate to the overlapping area of the gate line 6 and the drain line 26 through an interlayer insulating film 9, and the number of intersections between such gate lines 6 and drain lines 26. As shown in FIG. 18, however, no counter electrode is present under the drain line 26 in a region close to a power source, i.e., a region close to the drain driver region, and hence no additional capacitance is formed.
A s shown in FIG. 18, display pixels which are connected with the TFTs serving as display driving elements and a peripheral driving circuit consisting of drain drivers and gate drivers for driving the display driving elements are formed on the same substrate in a driver-integrated display.
FIGS. 19(a) to 19(f) are sectional views, taken along a line IV--IV in FIG. 3 described later, showing steps of fabricating the conventional liquid crystal display shown in FIG. 18.
&lt;Step 1: FIG. 19(a)&gt;
Polycrystalline silicon films 2 are formed on an insulating substrate 1 by LP-CVD (low-pressure chemical vapor deposition). The polycrystalline silicon films 2 are patterned in the form of islands, and separated into regions for forming a storage capacitance part, a TFT part and a drain driver.
&lt;Step 2: FIG. 19(b)&gt;
Gate insulating films 3 are formed on the polycrystalline silicon films 2 by LP-CVD, under film forming conditions of O.sub.2 gas and SiH.sub.4 gas at a ratio O.sub.2 /SiH.sub.4 of 5 to 200, a film forming temperature of 400 to 450.degree. C., and pressure of 1 Torr.
Then, resist films 4 are formed on the gate insulating films 3 except regions to be subjected to injection of phosphorus (P) by ion implantation later.
&lt;Step 3: FIG. 19(c)&gt;
Phosphorus (P) is injected into the polycrystalline silicon films 2 by ion implantation masked by the resist films 4, to form n.sup.+ -type polycrystalline silicon films.
Then, the resist films 4 are removed, and gate electrodes 5 and 8 are formed on the gate insulating films 3 for TFTs t1 and t2 of a display driving element and the drain driver respectively, while a gate line 6 is formed on the substrate 1. The electrodes 5 and 8 and the gate line 6 are prepared by forming polycrystalline silicon films by LP-CVD, forming tungsten silicide films on the polycrystalline silicon films, and thereafter etching the same by photolithography.
The polycrystalline silicon films forming the TFTs t1 and t2 of the display driving element and the drain driver respectively define active layers, and drain regions 12 and 14 and source regions 11 and 13 are formed on both sides of the gate electrodes 5 and 8 respectively.
&lt;Step 4: FIG. 19(d)&gt;
An interlayer insulating film 9 is formed to cover the overall substrate 1.
&lt;Step 5: FIG. 19(e)&gt;
Contact holes 10 are formed in the interlayer insulating film 9, by anisotropic etching. These contact holes 10 are formed in the drain regions 12 and 14 and the source regions 11 and 13 of the TFTs t1 and t2 of the display driving element and the drain driver.
Then, a pixel electrode 17 consisting of ITO is formed on the interlayer insulating film 9 covering a pixel part, by sputtering.
&lt;Step 6: FIG. 19(f)&gt;
An alignment film 18 is formed on the overall region of the substrate 1 including a storage capacitive electrode 27 and the TFTs t1 and t2 of the display driving element and the drain driver formed through the aforementioned steps. Further, a common electrode 20 consisting of ITO or the like and an alignment film 21 are successively formed on an opposite counter substrate 19.
Portions around the peripheries of the substrate 1 and the counter substrate 19 are pasted to each other with a sealing agent 22. The inside enclosed with the sealing agent 22 defines a display pixel region. This region is filled up with liquid crystals 23, thereby completing the liquid crystal display.
Characteristic problems of the conventional liquid crystal display which is fabricated in the aforementioned manner are now described.
First, the relation between a driving waveform and light transmittance in a liquid crystal display of an NW (normally white) mode is described. In the NW mode, the liquid crystal display has higher light transmittance upon application of a low voltage to liquid crystals as compared with that upon application of a high voltage. When twisted nematic (TN) liquid crystals are employed, for example, the liquid crystal display can be brought into the NW mode by orthogonalizing the axes of polarization of two polarizing plates which are arranged on both sides of the liquid crystal display to each other.
FIG. 20 illustrates the light transmittance of the conventional liquid crystal display of the NW mode. When a scanning signal Vg is applied to gates of TFTs serving as display driving elements which are arranged in the form of a matrix every horizontal scanning period, a row of TFTs conduct.
Then, consider that the amplitude of a video signal Vd is reduced in an intermediate stage, as shown in FIG. 20. Referring to FIG. 20, the video signal Vd which is transmitted to pixel electrodes through sources of conducting TFTs from drain lines of the TFTs has a large amplitude up to an (i+1)-throw, and a small amplitude after an (i+2)-th row. When the amplitude of the video signal Vd is thus reduced in the intermediate stage, the light transmittance T is not immediately increased, as shown in FIG. 20. Namely, a portion over several rows from that having the abrupt change of the video signal Vd, i.e., from the (i+2)-th row to an (i+4)-th row, is grayly displayed in the conventional liquid crystal display. Such a phenomenon is called a smear phenomenon. When the smear phenomenon is caused over several rows to tens of rows, the screen of the liquid crystal display appears contaminated, while visibility such as the contrast, color reproducibility and resolution of the liquid crystal display are damaged.
The aforementioned smear phenomenon is conceivably caused by the following reason: The capacitances which are formed between the drain lines of the display region and the common electrode provided on the counter substrate and between the drain lines and the gate lines are small, and these capacitances are readily changed by voltages which are applied to the drain lines due to the presence of the liquid crystals between the same and the common electrodes. When a video signal (black signal) having a large amplitude is written in the liquid crystals, therefore, the aforementioned smear phenomenon takes place to gradually whiten the display from gray over several rows to tens of rows if the transverse direction of black display, i.e., a scan direction of a gate signal (from left to right when the gate signal is inputted from the left side in the display, for example) is in white display.